Steam turbines generally have a rotor which is designed as a hollow body (drum rotor), as a solid body (monoblock), or as a body made up of individual discs (bodies of equal strength) welded together. This rotor body then carries the blades on its periphery, the blades usually being firmly clamped mechanically, or wedged in some manner. Since the mechanical and thermal demands made on the different parts vary, they are usually produced from different materials. The clamping positions on the rotor surface are then found to be particularly critical. At these points, either high temperature, corrosion resistance and/or stress corrosion resistance are required, depending on the location. Conventional rotor bodies--independent of their geometrical construction--generally consist of a single material, usually heat treated throughout. For this purpose, low to medium alloy steels are usually employed. It follows, therefore, that the mechanical, thermal and chemical loads occurring in operation would make it very desirable to have staging of the material properties, particularly in the radial direction.
It has already been proposed that turbine rotors be manufactured completely or partially of weld material, spiral layers being applied by welding runs on a relatively thin body of rotation--"build-up welding" (see DE-B-2 320 186). Protective surfaces produced by weld applications are also known (see, for example, EP-A-0 114 893).
Up to now, welding technology has been used for fastening blades only in the manufacture of impulse wheels. In this case, the blades are individually welded directly onto the rotor body. On the other hand, specialists have avoided the mechanical fastening of turbine blades in weld material because its properties were not sufficient for the chemical attack or mechanical loads at the temperatures involved. For the deposit welding described above, steels with a low C content were always used in the interests of good weldability. However, these steels have practically no hot strength. The same applies to the locations in the turbine where high mechanical strength and resistance to stress corrosion in condensed steam is demanded at low temperatures. Here again, no feasible way of using welding technology has been demonstrated (prejudice against fastening blades in weld material).
The objective of the invention is to provide a method of fastening blades on the periphery of a rotor body, the method ensuring optimum utilization of the properties of the materials used to meet the differing loads corresponding to the various locations, and being equally suitable for new manufacture and for repairs to existing rotors. The intention is that the method should be simple, involve less complication and be economical.